1. Field of the Invention
The present invention relates to a method and system for maneuvering a movable object and more particularly to a method and system for automatically track-keeping a movable object so as to move along a planned route.
2. Related Background Art
One known example of automatic movable object maneuver methods is a marine track-keeping method. For instance, Japanese Patent Publication Kokai Gazette No. 2001-287697 discloses an track-keeping method. This track-keeping method is carried out in the following way: a destination point is set at a position a specified distance away from the present location on the planned route; course and heading deviations are calculated from the destination point; and thrust that needs to be output from a plurality of propellers (hereinafter, referred to as “actuators”) is calculated from the calculated deviations. The route is a continuous planned ship course constituted by a plurality of line segments and a circular arc having a certain radius.
It, however, is necessary for this track-keeping method to momentarily calculate the destination point and to calculate a continuous route from given passing points beforehand. Further, since the ship is controlled so as to decrease the deviations from their respective target values, the ship travels while generating a necessary power so as to decrease the deviations even in disturbances such as winds and currents, involving the ship. This consumes a great amount of power in some cases.
Another known marine track-keeping method is disclosed in Japanese Patent Publication Kokai Gazette No. 07-242199. According to this track-keeping method, during an operation for changing the ship course from the present course to a new one, the ship turns with a fixed turn rate irrespective of a deviation from the present ship course and takes the new course upon completion of the turn. This track-keeping method controls in different manners in the course keeping operation and in the course change operation so that switching of control is necessary. In addition, since a turn with a fixed turn rate (i.e., feed forward control) is made in the course change operation, there arises the possibility that the deviation from the desired ship course increases.
A position control method for a movable object is disclosed in the Forty-fourth Automatic Control Association Lecture Meeting Gazette No. 01-253, pp 116 to 119. This position control method for a movable object teaches position control of a hover craft by the non-linear optimizing feed-back control, but does not discuss maneuver in consideration of constraints to a planned route. Another position control method for the same movable object is disclosed in “Measurement and Control” Vol. 36, No. 11 (November, 1997), pp776 to 783. This movable object position control method employs a maneuver technique for avoiding obstacles according to which penalty functions associated with the position of a movable object are included in a performance index. This technique is useful where the advent of obstacles is predictable, but cannot cope with situations where obstacles unexpectedly appear or move because the functions are time invariant. In addition, the penalty functions evaluate only the condition at a future time that is to come after an elapse of a time period T from the present time, and therefore, it can be hardly said that the avoidance of obstacles is always done on the optimum route.